Initialize the Output Layer.
Arguments:
o num_nodes -- The number of nodes in this layer. This corresponds
to the number of outputs in the neural network.
o activation -- The transformation function used to transform
predicted values.

Overrides:
object.__init__

update(self,
previous_layer)

backpropagate(self,
outputs,
learning_rate,
momentum)

Calculate the backpropagation error at a given node.
This calculates the error term using the formula:
p = (z - t) z (1 - z)
where z is the calculated value for the node, and t is the
real value.
Arguments:
o outputs - The list of output values we use to calculate the
errors in our predictions.